Superconducting dynamoelectric machine having a liquid metal shield

ABSTRACT

A superconducting synchronous motor or generator which utilizes separate radiation and damper shields around a superconducting field winding on the machine rotor. The damper shield includes a pair of concentrically disposed cylinders held in radial spaced relationship with each other by pillar type structural members. A liquid metal fills the space between the concentric cylinders and as the rotor, including the cylinders, is accelerated to its operating speed, the liquid metal likewise will reach synchronous speed as a result of viscous drag between the liquid metal and the cylinder walls. During operation, the rotor shielding function is performed by the electrically conducting liquid metal moving at synchronous speed during steady state operation. Current generated in the liquid metal shields the superconducting field winding from alternating flux generated in the armature due to phase imbalance and harmonics. During fault conditions, such as a terminal short circuit, currents are generated in the liquid metal to shield the high armature demagnetizing flux from the superconducting winding. These currents in the liquid metal react with the field flux in the air gap to produce body forces in the liquid metal which result in the motion of fluid particles. Forces moving in a radial direction would tend to establish a pressure gradient in the liquid metal, but since the liquid metal is free to move and the pressure gradient cannot be maintained, flow takes place in the space between cylinders to equalize the pressure in the annulus which contains the liquid metal. Also, since circumferential body forces can transmit shear only by viscous forces, the torque reaction is significantly reduced.

BACKGROUND OF THE INVENTION

The invention described herein relates to dynamoelectric machines andmore particularly to an improved damper shield used with superconductingsynchronous motors or generators.

The field windings of superconducting electric machines conventionallyare mounted on the machine rotor, and are enclosed in both aconcentrically disposed radiation shield and a damper shield. Theserotating shield elements are joined to form part of a dewar system whichmaintains the field winding under a vacuum at 4.2° K (-452° F), toachieve unusually high machine efficiency and performance. The radiationshield which includes both radial and axial members is placed betweenthe warm and cold portions of the machine to reduce the direct thermalradiation from the stator and other supporting structure operating atambient temperature to the lower temperature components on the rotor.The damper shield concentrically mounted outwardly from the radiationshield and is normally maintained at or near room temperature. It servesthe dual function of providing restoring torque to the rotor as a resultof load changes, and of shielding the field winding from ac fieldsgenerated in the stator from penetrating the low temperature 4.2° Kzone, during steady state and transient conditions to minimizeundesirable losses. Since the costs for removing these losses bycirculating liquid helium through the machine are relatively high, thedamper shield is made of a material having high electrical conductivityto provide the protective screening function.

Under circumstances of a terminal short circuit on a superconductingthree phase machine, the damper shield can be subjected to radiallycrushing forces which results from the interaction of armature flux andthe shielding flux generated by current induced in the shield. Theseforces which may act on the shield are illustrated in FIG. 1 andrepresent those forces resulting from a full three phase short circuit.The forces P₁ and P₂ for simplicity purposes, are shown as concentratedforces although it will be understood such forces are actually forcesdistributed around the rotor that vary as sin² θ. As shown, force P₁remains fixed with respect to the rotor surface while force P₂ movesaround the rotor at twice synchronous speed. The combined effect offorces P₁, P₂ create radially directed crushing forces which may reachvalues as high as 5000 psi, and cause the damper shield and its supportstructures to deflect and create high bending stresses in both thesupport and cylindrical structure. The most important disadvantageresulting from structural deformation is that the deflection couldresult in contact between the low temperature radiation shield and thefield winding which will introduce thermal losses that could result inthe loss of superconductivity. Such loss of superconductivity wouldrender the generator useless from both efficiency and performancestandpoints. The high stresses in the support structure could alsoresult in plastic deformation that may result in complete structuralfailure with consequent excessive damage to the machine.

In addition to the radial crushing forces P₁, P₂ discussed above, atorque T, is also developed in the shield that varies as a damped sinewave following the fault. Depending on the machine stability, such peaktorques as may be developed can be as high as 10 times rated machinetorque. Since these high oscillating torque values can be reached, astronger, heavier drive shaft for the machine is required to preventdamage during fault conditions.

SUMMARY OF THE INVENTION

Briefly stated, the above disadvantages are overcome by utilizing aliquid metal damping shield instead of the cylindrical metallic dampershield of the prior art. The shield liquid metal is contained in theannular space defined by a pair of concentrically disposed cylinderswhich enclose the rotor field winding. The cylinders therefore rotatewith the field winding and the liquid metal accordingly reachessynchronous speed. At this speed, the rotor shielding function isperformed by the electrically conducting liquid metal in which currentsare generated during both steady state and fault conditions. Thereaction of these currents with field flux causes the generation of apressure gradient in the liquid metal which moves the liquidcircumferentially in the shield annulus to equalize the pressuretherein. These forces likewise generate tangential body forces whichresult in oscillating liquid flow circumferentially in the damper shieldannulus.

It therefore is an object of the invention to provide a damper shieldhaving liquid metal contained therein useful with a superconductingelectrical machine.

Another object of the invention is to provide a liquid metal dampershield where the liquid metal therein responds to electrical forcesgenerated in the machine and causes the generation of liquid metalpressure gradients which results in equal distribution of pressure inthe damper shield annulus.

Still another object of the invention is to provide a damper shield fora superconducting electrical machine which contains liquid metal in thedamper shield which eliminates the reaction torques on the machine driveshaft when the machine is subjected to short circuit fault conditions atits terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of this invention is particularly pointed out anddistinctly claimed in the concluding portion of this specification. Theinvention, however, both as to organization and method of operation,together with further objects and advantages thereof may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which:

FIG. 1 is a schematic cross-section of a machine damper shield showingforce distribution.

FIG. 2 is a sectional view on elevation, partly in section, illustratingthe design of a superconducting synchronous generator.

FIG. 3 is a cross-sectional view in elevation, partly in section,showing the design of a rotor used with the generator of FIG. 1; and

FIG. 4 is a sectional view in elevation taken on line 4--4 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is shown in FIG. 2 a superconducting synchronous generator used toillustrate the teachings of the invention, although it will be apparentas the description proceeds that the invention is equally applicable toother designs and types of dynamoelectric machines. The machine includesa frame 20 arranged to support stator 22 and a rotor 24 axially disposedtherein. The stator includes a circumferential ring support 26 whichcontains a laminated iron core 28 held under compression by end rings30. An armature winding 32 located in the stator slots in a conventionalmanner is adapted for connection via terminals (not shown) to aconnected load.

The rotor 24 which is centrally disposed within the stator, includes adrive shaft 34 and input coupling 35 which transmits mechanical power tothe machine rotor. The drive shaft is mounted in bearings 36 on oppositeends of the frame 20. A field winding 38 mounted on the shaft peripheralsurface preferably is not disposed in slots as in conventional designs,but rather is attached directly to the rotor surface.

Referring more specifically to the rotor illustrated in FIG. 3, theshaft sections 40 disposed on opposite sides of the rotor rotate inbearings 36 appropriately supported from a base and terminate in flanges42 integrally formed with the shaft sections. These flanges are boltedor otherwise secured to the ends 44 of the body portion of rotor 24 bybolts 46. Rotor ends 44 are integrally formed with a cylindricallyshaped damper shield 48 which constitutes a wall for a dewar andrepresents the outer peripheral surface which rotates within stator 22.The complete area within the rotor structure is designed to operateunder a vacuum of about 10⁻⁵ Torr. A hollow cylindrical torque tube 50radially spaced from damper shield 48 extends axially inward from wall44 on the right side of the rotor 24 to provide a void space 51. Thetube terminates at its other end in a support member 52 which is held infixed relationship with the internal section of the rotor by a fieldsupport structure plate 54. A second void space 55 inside the torquetube is arranged to accommodate the field winding 38. As describedabove, axial and radial radiation shields 56, 58 are disposed around thefield winding 38 and the shield structure formed is supported byradiation shield support spokes 59 spaced along the rotating elementlength. Slip rings 60 mounted on the shaft surface in a conventionalmanner supplies electric power through conductor leads 62 to the rotorfield winding to provide the field excitation needed for machineoperation.

As illustrated more clearly in FIG. 4, the damper shield which islocated radially outward from the radiation shield 58, includes a pairof inner and outer cylinders 64, 66 concentrically disposed with respectto each other and separated by a number of pillars 68. The space 70defined by the concentric cylinders contains a liquid metal adapted tocirculate therein in response to both centrifugal and electromagneticforces which act on the liquid metal during machine operation. Since itis important that the liquid metal have complete freedom of circulation,the pillars 68 are of block-like configuration and do not extend asubstantial distance either axially or circumferentially of thecylindrical shells 60, 62.

As shown in FIG. 2, the superconducting field winding 38 is cooled by ahelium transfer system generally shown on the right side of FIG. 2.Although a number of different arrangements may be used in providing acoolant flow path through the rotor, one such conventional arrangementis to supply helium through an inlet 72 and associated helium transferequipment including pumps, valves, and the like, not shown, to thesuperconducting field winding located on the rotor. After the heliumabsorbs heat generated in the machine, it is returned to the transferequipment for recycling in the system in a manner well known in the art.

One type of conductor suitable for use in the superconductive fieldwinding is a fine filamentary niobium-titanium conductor twisted in acopper matrix. This type of conductor exhibits low eddy current andhysteresis losses when subjected to rapidly changing and alternatingfields of the kind generated in superconducting electrical machines.

These conductors comprising the field winding are mounted on the rotorin a manner which permits helium flow therethrough in an axial directionprior to being returned through a helium discharge outlet for return tothe source. Alternatively, the superconductor winding may be immersed ina bath of the liquid helium operating near atmospheric pressure so thatas boiling occurs at the winding surfaces, the heat is dissipated by thelatent heat of vaporization. Regardless of the particular cooling systemused, the winding must be supplied with liquid coolant through atransfer system which consists of stationary and rotating members. Also,the electrical leads interconnected the slip rings and the field windingconsists of a multi-filamentary conductor which are insulated from asupport tube and are cooled by the helium exhaust gas.

As indicated above, the axial and radial radiation shields 56, 58 serveto reduce the direct thermal radiation from the ambient structures tothe cooled components and the low temperature system. These shields arecooled by exhaust helium at some optimum temperature, usually between20° and 100° K.

The damper shield serves the dual function of providing restoring torqueto the rotor when load system changes cause rotor swings, and to shieldthe superconducting field winding from AC field generated by statorwinding harmonics and negative sequence fields, during steady state andtransient conditions. Because of the high conductivity of copper andaluminum at low temperatures, very thin shields can be used and thelosses will remain small.

As the rotor is accelerated to synchronous or other speed, adherence ofthe liquid metal to the cylindrical walls forming the damper shieldannulus will result in the liquid metal reaching synchronous speed.Since this speed represents steady state operation, currents generatedin the liquid metal will shield the superconducting field winding fromalternating flux generated in the stator due to the phase imbalance andharmonics. During the more serious fault conditions, such as terminalshort circuits, the damper shield normally is subjected to the radialcrushing forces due to the interaction of the armature flux and theshielding flux generated by currents induced in the shield. However, thecorresponding forces in a liquid metal shield results in the motion offluid particles. Therefore, as the fluxes interact, currents aregenerated in the liquid metal which act to shield the armature highdemagnetizing flux from the superconducting winding. These currents inthe liquid metal react with the field flux in the air gap to producephysical forces in the liquid metal which result in the actualdisplacement of liquid metal in a circumferential direction in thedamper annulus. When the electromagnetic forces are applied in a radialdirection, an increase in pressure in the liquid metal normally wouldresult, but since the liquid metal is free to move, the pressuregradient cannot be maintained and flows will be induced to equalize thepressure in the annulus containing the liquid metal. The tangential bodyforces that applied an oscillating torque on the metallic damper of thekind used in the prior art, will cause the liquid metal to flow in anoscillating manner and circumferentially in the damper shield liquidmetal annulus.

The pillars provided between the inner and outer cylindrical shells,provides sufficient support structure to maintain the integrity of theannulus. As pressures are generated or developed in the annulus, theywill tend to compress the inner cylinder and expand the outercontainment cylinder. This radial support structure offsets the pressurerise in the annulus by developing tensile stresses as the inner andouter structures tend to separate.

Since the liquid metal can only transmit force by pressure and shear andsince large pressure gradients cannot be maintained in the annulus, theforces on the containment cylinders will constitute external andinternal pressures thus eliminating the bending stresses encountered inthe metallic shield of the prior art. Also, since the circumferentialbody forces can transmit the shear only by viscous forces, the torquereaction will be significantly reduced.

A variety of liquid metals may be used in the annulus such as mercury,gallium, indium, woods metal, sodium, potassium, or alloys of thesemetals. If the conductivity is not sufficiently high, metallic particlesmay be added since this technique has been successfully used in ferrofluids where magnetic particles are ground to small diameters in theorder of angstroms and essentially dissolved in a fluid.

It will be apparent that many modifications and variations are possiblein light of the above teachings. It therefore is to be understood thatwithin the scope of the appending claims, the invention may be practicedother than is specifically described.

I claim:
 1. A superconducting dynamoelectric machine comprising a frameenclosing a stator having an armature winding therein, and a rotorsupporting a superconducting dc field winding arranged for rotation insaid stator,means for supporting said rotor for rotation on bearings insaid frame, said rotor including a cylindrical member completelyenclosing said field winding, means on said cylindrical membersupporting the field winding, said means being of sufficient structuralstrength to absorb torque forces imposed on said rotor during operation,radiation shield means in said cylindrical member for absorbing thermalradiation from structural members operating in substantially ambienttemperature, said cylindrical member including damper shield means whichcontains an electrically conductive liquid which is effective inrestoring torque to the rotor when load system changes and in shieldingthe superconductive field winding from ac fields generated in the statorduring operation, and means in said shaft for supplying excitationcurrent to said winding and cooling means associated with said machinefor furnishing a fluid coolant to said rotor for maintaining the fieldwinding at about 4.2° K.
 2. The machine according to claim 1 wherein thedamper shield means comprises a pair of cylindrical members held inspaced relation to each other by spacer supports which determine thedistance between said members, and wherein said electrically conductiveliquid fills the space between said cylindrical members.
 3. The machineaccording to claim 2 wherein said electrically conductive liquidcomprises a liquid metal which during fault conditions on the machinegenerates a current in the liquid metal which reacts with field flux inthe air gap to produce forces on the liquid metal which causes it toflow and equalize the liquid metal pressure between the cylindricalmembers.
 4. The machine according to claim 1 wherein said damper shieldmeans comprises a pair of concentric cylinders held in spaced relationwith each other, said spaced cylinders forming a closed annulus,a liquidmetal in said annulus, said liquid metal being effective in performing adampening function by generating currents therein which preclude thetransmission of high flux forces from the stator to said superconductingwinding, said forces resulting in the establishment of pressurefluctuations and circulating flaws in the liquid metal.